1. Field of the Invention
This invention relates to a magnetic recording medium and the preparation thereof.
2. Description of the Prior Art
When iron-cobalt ferrites have many sites of cation vacancy, uniaxial induced magnetic anisotropy of the ferrites rather easily develops. Moreover, magnetic annealing treatment for this anisotropy has also been found. This phenomenon is believed to be caused by the diffusion of Co.sup.2.sup.+ ion, which results in the location of cobalt ion sites in proximity to the direction of the applied magnetic field in the spinel B sites as a result of the magnetic annealing effect. By this treatment, substances having high uniaxial induced magnetic anisotropy in the direction of the applied magnetic field have been prepared. It has been proposed to prepare magnetic tapes having improved squareness in the longtudinal direction of the tapes by application of the magnetic annealing treatment utilizing induced magnetic anisotropy.
However, the magnetic annealing treatment is only effective for the improvement of magnetic characteristics of magnetic tapes. On the other hand, the tapes treated by magnetic annealing have the disadvantage of having magnetic instability called relaxation phenomenon. Moreover, it is difficult in the mass production of magnetic tapes to generate a uniform magnetic field having a temperature gradient which can be used to effect the magnetic annealing treatment.
Among the disadvantages the relaxation phenomenon has appeared to result in the loss of improved magnetic characteristics of the magnetic tape because of the gradual decrease of the uniaxial anisotropy induced in the longitudinal direction of the tape by the magnetic annealing. The reason for this is that the thermal distribution of Co.sup.2.sup.+ ions at high temperature is fixed by an external magnetic field in the magnetic annealing treatment. However, if an external magnetic field is removed after the magnetic annealing treatment the thermal distribution of Co.sup.2.sup.+ ions changes to the original random distribution by thermal disturbance at room temperature. Thus, if the Co.sup.2.sup.+ ion sites can be fixed not to move at room temperature after the magnetic treatment, the induced anisotropy can be stabilized. In this regard, it has been proposed to stabilize the induced anisotropy by removing the cation vacancies of the substance after the magnetic annealing treatment. However, it is impossible to completely fix the Co.sup.2.sup.+ ions without completely removing the cation vacancies and even though the relaxation velocity can be decreased, gradual aging over long periods of time cannot be prevented by the oxidation of the substances. Moreover, the induced anisotropy may decrease by moving the Co.sup.2.sup.+ ions by removing cation vacancies. As a matter of fact, when all cation vacancies are removed, the induced anisotropy also disappears. Thus, the method of stabilization by removing cation vacancies may not solve the problem of the relaxation phenomenon.
There have also been disadvantages with the apparatus used in the magnetic annealing in the mass production of magnetic tapes. In the practical operation for magnetic annealing the tapes in the form of rolls prior to slitting, have to be maintained uniformly in the condition of magnetic saturation along the longitudinal direction from high temperature to low temperature (room temperature).
For this purpose, it is necessary to use a long solenoid having a large diameter which generates magnetic fields greater than 2000 Oe. Such devices for practical purposes have been difficult to construct. A need therefore, exists for a magnetic recording medium which has a high magnetic stability and high tape sensitivity, utilizing induced magnetic anisotropy.